Two novel mucin genes down-regulated in colorectal cancer identified by differential display

Epithelial mucins are large, secreted and cell surface glycoproteins involved in epithelial cell protection, adhesion modulation, and signaling. Using differential display, we have identified two novel mucin cDNAs (dd34 and dd29), hereafter designated MUC11 and MUC12, respectively, that are down-regulated in colorectal cancers. Northern blots demonstrated polydisperse signals characteristic of mucin transcripts in RNA from normal colon that were absent in colorectal cancer. Both cDNAs were mapped by fluorescence in situ hybridization to chromosome band 7q22, the location of the MUC3 mucin gene, thus suggesting that there may be a cluster of mucin genes at this locus. The sequences of both differential display clones were extended by a combination of screening libraries and PCR. The 2.8-kb MUC11 cDNA composite encoded 35 serine/threonine-rich, mucin-like degenerate 28 amino acid tandem repeats. The MUC12 cDNA composite encoded a putative transmembrane mucin containing two extracellular cysteine-rich, EGF-like domains, a coiled-coil region, and a mucin-like domain consisting of 28 amino acid degenerate tandem repeats. Distinct patterns of expression of MUC11, MUC12, and MUC3 mRNAs were observed in a range of normal human tissues. MUC12 mRNA was not expressed in any of six colorectal cancer cell lines examined and was down-regulated or absent in 6 of 15 (40%) tumors compared with matched normal colonic tissue. In contrast, MUC11 showed a different pattern of mRNA expression, with four of these lines showing low levels and the other two lines showing relatively high levels of MUC11 transcripts. Expression of MUC11 was down-regulated in the tumors of 12 of 15 (80%) paired samples. Structural homology of MUC12 with rat, mouse, and human MUC3 and human and rat MUC4/ASGP2 indicate that there is a distinct subfamily of transmembrane mucins with conserved epidermal growth factor domains. The homology of MUC12 with epidermal growth factor-like growth factors and its down-regulation in colorectal cancers, together with known interactions between rat MUC4 and c-erbB-2 growth factor receptors, suggests that MUC12 may be involved in epithelial cell growth regulation.